Constitutive STAT3 signaling participates in tumorigenesis by stimulating cell proliferation, mediating immune evasion, promoting angiogenesis, conferring drug and radiation resistance, and tumorigenesis. The persistent STAT3 activation is frequently detected in human medulloblastoma cell lines and primary tumors of the most frequently occurring malignant brain tumor, medulloblastoma, in children. The central hypotheses of this project are: STAT3 phosphorylation is expressed in cancer tissues in four medulloblastoma groups and targeting persistent STAT3 signaling using LY5 in combination with cisplatin and radiation therapy is an effective approach for medulloblastoma therapy. The hypotheses are supported by our recent data demonstrating that STAT3 phosphorylation is expressed in medulloblastoma cell lines and primary tumor samples. LY5 inhibited cell viability and induced apoptosis of human medulloblastoma cell lines but has little toxicity in normal human cells and tumor-free normal immunocompetent mice. In addition, we observed that combination of LY5 with cisplatin or irradiation exhibited stronger inhibitory effect in medulloblastoma cells. Furthermore, our preliminary the pharmacokinetic data showed that LY5 has high oral bioavailability and good blood brain barrier penetration. The objectives of this proposal are to build on these initial findings to further understand the upstream signaling responsible for STAT3 activation in medulloblastoma, STAT3 activation in different subgroups, and to evaluate the biologic activity of combinational treatments in medulloblastoma mouse models. Our long-term objective is to move a STAT3- selective inhibitor such as LY5 into clinical evaluation in patients as a STAT3-targeting drug for medulloblastoma therapy. We will test the central hypotheses through the following specific aims: (1) Characterize STAT3 phosphorylation in four subgroups of medulloblastoma and investigate the mechanisms responsible for STAT3 activation. (2) Characterize the biologic effects of the novel STAT3 inhibitor LY5 on medulloblastoma cells. (3) Evaluate the inhibitory efficacy of LY5 in mouse medulloblastoma models in vivo.